Activated protein C (APC) is an important regulatory protein that helps maintain the balance of pro-coagulant and anti-coagulant properties within the circulation. In acts by cleaving activated factor V and activated factor VIII, thereby limiting clot formation at the appropriate level. An abnormal factor V that is resistant to cleavage by APC has been described recently in patients with venous thrombosis. We have set up an assay using DNA amplification and restriction enzyme analysis of the portion of the factor V gene responsible for its resistance to APC. Seven patients have been assessed and appropriate clinical treatment has been instituted as a result of these test results.